Recently, a wireless ad hoc network (self-configuring network) linking plural wireless stations by wireless communication is under investigation. In the wireless ad hoc network, a transmitting wireless station transmits a packet to a receiving wireless station and confirms the success of the transmission by receiving a response signal (Ack) transmitted from the receiving wireless station.
If no response signal has been received even after a given waiting period has elapsed, the transmitting wireless station judges the transmission to be unsuccessful and retransmits the packet. If an optimum pathway and an alternative pathway are present for one address, and no response signal is received even after the packet has been retransmitted a number of times to the wireless station via the optimum pathway, the transmitting wireless station switches the pathway and transmits the packet to the wireless station via the alternative pathway.
Depending on the radio wave frequency to be used (e.g., 950 [MHz] band), there is a legal regulation of setting a transmission prohibition period during which signal transmission is prohibited after the transmission by the wireless station. When each wireless station has an active period in which transmission/reception is possible and a sleep period in which the transmission/reception is not possible, a technology is known of synchronizing so that each wireless station will have an active period common with an adjacent wireless station (see, e.g., Japanese Laid-Open Patent Publication No. 2009-296215). The wireless stations perform synchronization, for example, by transmitting/receiving a synchronization packet periodically.
In the wireless ad hoc network, since the transmitting timings of the wireless stations are asynchronous and the transmission prohibition periods of the wireless stations are also asynchronous, a packet may be transmitted to a wireless station during the transmission prohibition period. In this case, even if the packet is successfully received, the receiving wireless station cannot transmit a response signal and as a result, the transmitting wireless station retransmits the properly received data. Consequently, a problem arises in that traffic increases and throughput of the network decreases.
With the technology of synchronizing wireless stations by periodic transmission of synchronization packets, a problem arises in that the synchronization packets increase traffic and lower network throughput. Since the transmission prohibition period is set so that the wireless stations will synchronize with each other, the transmission prohibition period becomes longer than regulated, resulting in a delay in the transmission of the packet and lowered throughput.